exactextractr
This package provides routines to perform fast and accurate zonal statistics on raster and sf objects in R using the exactextract library.
Relative to other implementations, such as the extract function in the raster package, it:
- computes the exact percentage of each raster cell that is covered by a polygon
- avoids the use of computationally expensive raster disaggregation or polygon clipping methods
Additional background and a description of the method is available here.
Example
require(raster)
require(sf)
require(exactextractr)
# Pull some administrative boundaries for Brazil
brazil <- st_as_sf(getData('GADM', country='BRA', level=2))
# Pull some climate data (max. temperature)
temp <- getData('worldclim', var='tmax', res=10)[[12]]
# Find the mean-max temperature for each administrative boundary
brazil$max_dec_temp <- exact_extract(temp, brazil, weighted.mean, na.rm=TRUE)
plot(brazil['max_dec_temp'])
# Output a matrix of cell values, cell coordinates, and coverage fractions for a given polygon
exact_extract(temp, brazil[1, ], include_xy=TRUE)
# Generate a raster showing cell coverage fractions for a given polygon
can <- st_as_sf(getData('GADM', country='CAN', level=0))
plot(partial_mask(temp, can)[[1]])
Dependencies
Installation requires the GEOS geometry processing library.
For best performance, it is recommended to use version 3.7, which introduced some optimizations important to exactextractr.
On Windows, GEOS will be downloaded automatically as part of package install.
On MacOS, it can be installed using Homebrew (brew install geos).
On Linux, it can be installed from system package repositories (apt-get install libgeos-dev on Debian/Ubuntu, or yum install libgeos-devel on CentOS/RedHat.)
On Windows, the R build tools (Rtools) are also necessary.
Installation
exactextractr is not in CRAN and can be installed from source only. Using devtools, the package can be installed with:
devtools::install_github('isciences/exactextractr')Limitations
- Raster and polygon inputs must be in the same coordinate reference system.
- The portion of the raster that intersects any given polygon must fit into memory. (The entire raster does not need to fit into memory.)